TinySCHEME being a SECD-machine needs to push frames onto the dump
stack. Previously, the dump stack was a list. This required four
cells for the spine, as well as up to one additional cell to encode
the current opcode. This was quite inefficient despite the fact that
we recovered the spine as well as the integer cell.

We introduce frame objects, which are a special variant of vectors of
length four. Since the length is fixed, this frees up the length
field of the vector object to store the unboxed opcode. A frame
object now fits in two cells.

Saving two or three cells is a mere byproduct, the performance gain
comes from increased locality, unboxed opcode representation, and the
ability to easily put the objects in a free list, keeping the garbage
collector out of the continuous motion of the virtual machine.